PROBLEM: `vim.treesitter.get_node()` does not recognize the `lang` in
the option table. This option was used in somewhere else, for instance,
`vim.treesitter.dev` (for `inspect_tree`) but was never implemented.
SOLUTION: Make `get_node()` correctly use `opts.lang` when getting a
treesitter parser.
Unfortunately the gc=false objects can refer to a dangling tree if the
gc=true tree was freed first. This reuses the same tree object as the
node itself is keeping alive via the uservalue of the node userdata.
(wrapped in a table due to lua 5.1 restrictions)
Problem: `push_tree`, every time its called for the same TSTree with
`do_copy=false` argument, creates a new userdata for it. Each userdata,
when garbage collected, frees the same TSTree C object.
Solution: Add flag to userdata, which indicates, should C object,
which userdata points to, be freed, when userdata is garbage collected.
